Response Characteristics of Muscle Spindle Endings at Constant Length to Variations in Fusimotor Activation

The aim of the present investigation was to determine the transfer function between fusimotor stimulus rate and afferent impulse frequency of muscle spindles at constant length. Spindle endings in the external intercostal and in the ankle extensor and flexor muscles of the cat have been subjected to variations in fusimotor stimulation frequency. The variations had sinusoidal form and ranged in repetition rate from 1/4 to 1/12 c/s. Both single‐fibre and double‐fibre stimulations were studied. Linear static and dynamic input‐output relations were found in the stimulus frequency range 60 to 180 pulses/sec. For all combinations of one or two fusimotor fibres to one spindle ending the transfer function had the same general form. The static sensitivities were largest for primary endings under static fusimotor control; about the same sensitivities were obtained in dynamic fusimotor activation of primary endings and in static activation of secondary endings. With regard to dynamic properties all combinations of fusimotor fibres and spindle endings were equivalent. This was further tested with step and triangular wave inputs. Explanations for this similarity are discussed.